1. Field of the Invention
This invention relates to trip circuits and more particularly to a novel and highly effective trip circuit having a first portion with a normal, relatively slow reaction time for opening a protected circuit in response to detection of the rated trip current level in the protected circuit and also having a second portion with a very fast reaction time for opening the protected circuit very quickly in case the current level in the protected circuit reaches a dangerously high level before the first portion can react.
2. Description of the Prior Art
Devices for protecting against overload of a circuit have been known virtually as long as electric circuited themselves. The earliest and simplest such devices are fuses that break down, as by melting, in response to a current level above some rated level, called the trip level, which is judged to be the highest current level that the protected circuit can safely handle. Fuses operate reliably enough for many applications but always irreversibly. That is, once a fuse has "blown" it cannot thereafter pass a current and must be replaced in order to restore the protected circuit to operation. The replacement process is normally a manual one and therefore relatively slow, labor-intensive and expensive.
Circuit breakers are an improvement on fuses in several respects. A major advantage of circuit breakers is that they operate reversibly and can be reset (manually in some cases and automatically in other cases) without being physically replaced.
However, even the best modern circuit breakers fail to operate ideally under certain circumstances. In particular, there is a tradeoff between the sensitivity of a trip circuit and its reaction time. Thus the lower the current level at which the circuit breaker is intended to operate, the more sensitive the trip circuit for tripping the circuit breaker must be and therefore, other factors being constant, the slower its reaction time will be. For example, a typical trip circuit for a circuit breaker designed to operate at a current of 100 amps may include a coil having on the order of 13,000 or 14,000 turns and a reaction time on the order of 10 milliseconds. The coil is actually connected to a separate power supply of about 7.5 amps max at 60 volts max and to a normally open switch such as a silicon-controlled rectifier gated by a microprocessor. In a typical industrial installation, the microprocessor may be connected to one or more of three secondary transformer coils of a three-phase main power supply for, say, a factory. The circuit breaker is normally connected in the primary side of the main power supply so that when it is tripped it shuts off all three of the secondary coils.
The large number of turns in the coil mentioned above (13,000 or 14,000) is necessary in order to give the trip circuit enough sensitivity to perform its function. However, the large number of turns corresponds to an increase in the inductance of the coil, which increases the amount of energy required to activate the coil and increases the amount of time required for the activation. The problem that sometimes arises is that, before the coil can react to trip the circuit breaker, the current through the circuit breaker continues to build to a dangerously high level. For example, a circuit breaker that should trip at 100 amps may fail to trip before a current of thousands of amps begins to flow through the circuit breaker, No adequate remedy for this problem exists in the prior art.